1. Field of the Invention
This invention relates to a method of transmitting digital data, such as is known from an article by Marvin K. Simon and Andreas Polydoros, entitled "Coherent Detection of Frequency-Hopped Quadrature Modulations in the Presence of Jamming-Part I: QPSK and QASK Modulations", published in IEEE Transactions on Communication, Vol. COM-29, No. 11, November 1981, Pages 1644 and following.
2. Discussion of Background
As a modulation process, the so-called "Phase Shift Keying" (abbreviated to PSK) is used in the article mentioned. This is a modulation process with very favorable power characteristics, for example in comparison with the so-called "Frequency Shift Keying" (abbreviated to FSK), but which, as a coherent process, requires in each case the accurate knowledge of the phase of the carrier wave during the detection of the transmitted data in the receiver. Owing to the continuous sudden change in frequency of the carrier wave, also called "Frequency Hopping" (abbreviated to FH) in communications engineering, in order to evade narrow-band interference sources in the transmission path, the phase coherence can be lost. For this reason, the phase of the carrier wave must, as a rule, be re-determined between two frequency jumps in each case in each transmission section. In the article mentioned, this phase determination is pointed out as a difficult problem. Fundamentally, this problem could be solved by transmitting special synchronization information, for example in a preamble, in addition to the data in each transmission section. But this solution is very elaborate and therefore not very practicable.